Abstract:

Improved methods for manufacturing silicon carbide rings using chemical
vapor deposition. Cylindrical tubes are used as deposition substrates and
the resulting material deposited on the inside surface of cylindrical
tubes or on the outside surface of cylindrical mandrels, or both, is
sliced or cut into the desired ring size and shape. The resulting rings
have a crystal growth that is oriented substantially planar to the
finished article. The invention also relates to nitrogen doped silicon
carbide material, as well as to silicon carbide structures having axes of
grain growth substantially parallel to the plane of the structure and to
each other, and having rotational orientation that is substantially
random with respect to the axes of grain growth of the grains.

Claims:

1.-14. (canceled)

15. A method of making structures formed by chemical vapor deposition,
comprising:(a) forming material by chemical vapor deposition on a
surface(b) dividing the material by slicing along a direction such as
will form one or more structures having a dimension in the planar
direction that is larger than the dimension in the normal dimension and
having grains substantially oriented in the planar direction.

16. The method of claim 15, in which the structures comprise flat rings,
the method comprising:(a) providing a round cross-sectional tube in a
deposition zone;(b) directing precursor gas into the tube;(c) chemically
reacting the precursor gas to form a solid deposit in the shape of the
tube on the inside on the tube;(d) removing the solid deposit; and(e)
dividing the solid deposit into substantially flat rings either before or
after removal.

17. The method of claim 16, further comprising directing precursor gas
onto an outer surface of the tube, chemically reacting this precursor gas
to form a solid deposit on the outside of the tube, removing this solid
deposit, and dividing this solid deposit into substantially flat rings
either before or after removal.

18. The method of claim 15, in which the structures comprise flat rings,
the method comprising:(a) providing in a deposition zone a mandrel having
a substantially round cross-sectional;(b) directing precursor gas onto
the outer surface of the mandrel;(c) chemically reacting the precursor
gas to form a solid deposit approximately in the shape of the mandrel on
the outside of the mandrel;(d) removing the solid deposit; and(e)
dividing the solid deposit into substantially flat rings either before or
after removal.

25. The method of claim 15, wherein the ring has a planar direction and a
normal direction with the dimension in the planar direction being larger
than the dimension in the normal dimension, and wherein grains of the
ring that are deposited by chemical vapor deposition are substantially
oriented in the planar direction.

26. The method, of claim 15, wherein the ring has a circumference and
wherein the grains are oriented in a substantially radial direction
around the circumference of the ring.

27. The method of claim 15, wherein several tubes are positioned within a
CVD furnace.

28. The method of claim 15, further comprising applying a coating of
silicon to at least one surface of the ring.

29. The method of claim 28, further comprising machining the ring prior to
applying the silicon coating.

30. The method of claim 28, wherein the silicon coating is applied using
chemical vapor deposition.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]This application claims priority to U.S. patent application Ser. No.
10/729,262 filed on Dec. 5, 2003, the contents of which are incorporated
herein by reference.

BACKGROUND

[0002]1. Field of the Invention

[0003]The invention relates to articles formed by chemical vapor
deposition and methods of forming such articles.

[0004]2. Description of Related Art

[0005]Chemical vapor deposition (CVD) techniques have been widely used to
provide thin films and coatings of a variety of materials on various
products. Typically, the process involves reacting vaporized or gaseous
chemical precursors in the vicinity of a substrate to result in a
material such as silicon carbide (SiC) depositing on the substrate. The
deposition reaction is continued until the deposit reaches the desired
thickness.

[0006]CVD techniques can be used to form relatively thin coatings on the
surfaces of pre-existing articles; in this situation, the surface of the
article forms the substrate. However, CVD techniques can also be adapted
to produce articles that are formed from the deposited material. In this
situation, the substrate upon which deposition occurs is a form or
mandrel that provides an initial shape to the article. The article, which
is removed after sufficient deposition has occurred, has a complementary
surface that corresponds to the form or mandrel. Such articles are called
"free-standing" articles herein.

[0007]One method by which free-standing SiC articles are formed by CVD
includes feeding silicon carbide precursor gases or vapors into a
deposition chamber, where they are heated to a temperature at which they
react to produce silicon carbide. The precursor gases or vapors react at
the surface of a substrate or other structure loaded into or placed in
the chamber. The silicon carbide builds up as a shell or a deposit on the
substrate. Different articles may require different thicknesses, and
thicknesses can range from less than 100 microns to over an inch or two
inches thick. The thickness can be controlled by controlling the
deposition time and/or other process variables. When the desired
deposition thickness is reached, the mandrel or substrate is then removed
from the deposition chamber and the deposited silicon carbide is
separated therefrom.

[0008]In one method for forming SiC articles, methyltrichlorosilane
(CH3SiCl3 or MTS), hydrogen (H2), and argon (Ar) gases are
introduced into the furnace through an injector. MTS is a liquid at
ambient or room temperature, and sufficient vapor can be delivered into
the reaction vessel by feeding carrier gas through the MTS liquid or by
picking up vapor above the liquid. Gases that are unreacted in the
furnace are pumped out by a vacuum pump, filtered, and cleaned in a gas
scrubber before being vented to the atmosphere.

[0009]Some industries require thinly-formed silicon carbide rings or
articles. The current technology of producing monolithic ceramic parts
via the chemical vapor deposition process includes producing large sheets
of the ceramic material in a CVD furnace. SiC is deposited onto a flat or
box-shaped substrate to form the relatively large, flat sheets, from
which the final ceramic part or parts are machined. The machining process
includes cutting the rough shape out of the large sheet, grinding the
piece to near the desired thickness to produce a blank that approximates
the final form but with surplus material thickness on each face, and then
machining the blank to the dimensions of the final form.

[0010]When CVD materials are deposited on large flat substrates, e.g.
plates, the material exhibits a direction of crystal growth that is
perpendicular to the plane of the flat substrate. The material is not
necessarily deposited evenly to form a sheet that is uniform in thickness
or in microstructure, so that there may be thicker portions in some areas
and thinner portions in other areas, instead of a uniform thickness
throughout. Because the as-deposited sheet typically has a thickness
profile that is non-uniform, the thickness of rings in rough shape form
can vary significantly, depending upon where they were cut from the
sheet. Moreover, there may be significant thickness variation within an
individual ring in rough shape form.

[0011]In addition, due to the large size of the SiC sheet from which the
SiC rings are typically formed, different microstructures may exist at
different areas of the sheet. For example, when a ring is produced from a
sheet of deposited material, differences in the material characteristics
across the thickness of the deposited sheet may lead to increased tension
or stress within the material that can cause a slight axial bow or curve
in the machined ring.

[0012]Another manifestation of the different microstructures across the
large plate is a variation in cosmetic appearance across the final
machined parts, e.g. rings. These variations in cosmetic appearance can
be further magnified when the finished parts are coated with other vacuum
deposited or vapor deposited coatings, especially CVD deposited coatings
such as silicon.

[0013]Additionally, because of the thickness variations, the CVD process
time must be increased to bring the low deposition rate areas up to the
minimum thickness requirements for the desired parts. The higher
deposition rate areas then cause the pieces cut from the sheet to require
more machining time in order to be ground to the required thickness.

[0014]Moreover, there is limited flexibility in the geometry of a sheet
and the pieces to be cut, so there is material waste due to the layout
pattern of the pieces. There is also surplus material in the piece
pattern that is cut or ground from between and within the pieces to make
the blanks, such as edges and centers of rings, which results in a large
quantity of scrap material. For example, the material that is cut out
around each ring and between each ring (to form an inner diameter of the
ring) is typically wasted, much like the unused cookie dough remaining
after cookies are stamped out from a roll of cookie dough.

[0015]There are also occasional problems with cracking of a large,
CVD-produced plates or sheets of ceramic material during the CVD process,
which can reduce the yield significantly. The combination of cracks and
the scrap material from the sheet and the material that is ground from
the blanks lowers the average raw material-to-product conversion ratio
significantly.

[0016]An alternate method that has been used to manufacture silicon
carbide rings includes mounting disk-like mandrel substrates through
their respective centers in a spaced and parallel relationship in grooves
on a shaft. The planar surfaces of the disk-like mandrel substrates are
oriented perpendicular to the axis of the shaft. During processing, the
shaft is rotated as gases are injected into the chamber, such that
ring-shaped ceramic parts are formed by deposition on the mandrel
substrates.

[0017]A different but related method of creating silicon carbide rings is
to suspend within a deposition chamber individual, flat graphite ring
mandrels having an outside diameter and an inside diameter similar to
those desired in the SiC rings. The gas mixture of MTS in hydrogen and
argon is fed into the chamber and silicon carbide is deposited on the
mandrels to form rings. Once the rings are removed from the graphite ring
mandrels, their inner and outer diameter can be machined to the desired
dimensions.

[0018]One problem related to forming silicon carbide rings from these
alternative processes is that the mandrels need to be rotated throughout
the formation process to prevent build-up in undesired areas. In many
instances, the gases are injected into the reaction chamber such that the
gases are not focused on any particular mandrel or surface of interest,
but instead are allowed to deposit non-uniformly on all surfaces of the
reactor. The rotation or suspension of the substrate in the reaction gas
stream is intended to prevent or limit this non-uniformity of deposition.

[0019]The specifically-shaped mandrels are also complicated to
manufacture. In the embodiment that includes a shaft with disk-shaped
mandrels placed in grooves, the grooves must be specially machined for
receiving and supporting the disk-shaped mandrel substrates. In the
graphite ring embodiment, each ring support axis must have protrusions or
tabs that facilitate their suspension or placement in the chamber, and
material is often deposited in or around the protrusions or tabs.

[0020]Additionally, in each of the above-described prior art methods, the
resulting ring has a crystal growth that is oriented axially relative to
the ring or finished article, not radially oriented relative to the ring
or article. In other words, in each of the aforementioned prior art
methods, the microstructure contains grains having their long direction
oriented perpendicular to the plane of the finished part. Further, it is
known that as materials are deposited by chemical vapor deposition the
size of grains increases as the growth proceeds away from the substrate.
The evolution in grain structure from small grains to large grains occurs
as grains with lower energy crystallographic orientations grow faster
than grains with less preferred orientations, under the particular
deposition conditions used. This evolution of grain structure typically
produces a gradient in microstructure across the thickness of the
material, which in turn causes a gradient in internal stress of the
deposited material, resulting in bending or "bow" of the deposited
material when it is released from the substrate. This gradient in
material microstructure and stress complicates the machining process, and
remains in the material even after machining is completed, often
resulting in some bow or waviness in the finished part. This bow or
waviness is undesirable, especially in parts that require precise
tolerances and extremely consistent flatness, such as rings for use in
contact with semiconductor wafers. The larger the part, the more
significant the problem can become.

[0021]Regardless of these difficulties and costs associated with
manufacturing silicon carbide articles, silicon carbide has a unique
combination of properties that make it a particularly suitable material
for a variety of applications in the semiconductor, optical, electronic
and chemical processing fields. Silicon carbide articles produced by CVD
processing are recognized to exhibit superior chemical, mechanical,
thermal, physical and optical properties.

[0022]Some semiconductor processing apparatus, such as rapid thermal
processing chambers, require the use of thin, SiC-edge rings to support
Si wafers during high temperature processing. It is important that these
rings be opaque to light, despite being relatively thin, in order to
avoid causing irregularities in optical pyrometry temperature measurement
of the wafers. Typically, CVD SiC rings are coated with a layer of CVD Si
that is 50-100 μm thick to provide the ring with adequate opacity.
There is thus a need in the art for a CVD SiC edge ring material that has
an optical density closer to single-crystal Si than is possible with
standard sheet-form CVD SiC, in order to reduce or eliminate the need for
Si coating.

[0023]Accordingly, the present inventors have found a way to improve CVD
processing for producing articles, in particular planar articles such as
rings and discs, and more particularly silicon carbide ring-shaped
articles. The resulting articles have a unique microstructural
orientation relative to the shape of the article. The invention may also
be used to create articles having other shapes. Such articles can be used
in fixtures to support silicon and other wafers for processing, susceptor
rings for supporting wafers in semiconductor furnaces, and as wafer edge
rings.

SUMMARY OF THE INVENTION

[0024]The invention provides a structure formed by chemical vapor
deposition having a planar direction and a normal direction, wherein the
structure has a dimension in the planar direction that is larger than the
dimension in the normal dimension and having grains substantially
oriented in the planar direction.

[0025]The invention further provides improved methods for manufacturing
rings using chemical vapor deposition. In particular, the invention
provides a method of making such structures by:-- [0026]a) forming
material by chemical vapor deposition on a surface [0027]b) dividing the
material by slicing along a direction such as will foam one or more
structures having a dimension in the planar direction that is larger than
the dimension in the normal dimension and having grains substantially
oriented in the planar direction.

[0028]In certain embodiments, tubes (which may be cylindrical) are used as
substrates and the resulting material that is deposited on and then
removed from the inside or outer surface of the cylindrical tubes can be
sliced or cut into the desired ring size and shape. The resulting ring
has a diameter in a planar direction and a height in a normal direction.
The crystalline grains are oriented in the planar direction, and in
certain embodiments, are oriented radially in the planar direction, as
opposed to the perpendicular or axial growth orientations seen in the
prior art rings. Put another way, the primary direction of crystal growth
in materials according to the invention is in the plane of the resulting
article; in the case of ring, hoop, or disc shaped articles, the primary
direction of crystal growth and grain orientation is in the radial
direction.

[0029]The invention also relates to methods of forming disk-shaped rings
by chemical vapor deposition by

(a) providing a round cross-sectional tube in a deposition zone;(b)
directing precursor gas into the tube;(c) chemically reacting the
precursor gas to form a solid deposit in the shape of the tube on the
inside on the tube;(d) removing the solid deposit; and(e) dividing the
solid deposit into substantially flat rings.

[0030]Alternatively, the solid deposit can be formed on the outer surface
of a mandrel disposed in the deposition zone following steps analogous to
those described above. The mandrel upon which the outside deposit is
formed may also be a tube, so that solid deposit forms on both the inside
and outside surfaces of the mandrel, thereby forming two sets of rings of
different sizes.

[0031]In this respect, the invention is directed to a method of making
flat rings by chemical vapor deposition, comprising:

(a) providing in a deposition zone a mandrel having a substantially round
cross-section;(b) directing precursor gas onto the outer surface of the
mandrel;(c) chemically reacting the precursor gas to form a solid deposit
approximately in the shape of the mandrel on the outside of the
mandrel;(d) removing the solid deposit; and(e) dividing the solid deposit
into substantially flat rings either before or after removal.

[0032]Advantages of this invention include increased space utilization,
reduced unit cost of manufacturing the rings, reduced waste during
manufacture, and the ability to produce rings of varying diameter,
cross-sectional width and thickness depending on the height of cutting
(i.e., without varying the deposition time).

[0033]The invention is described in more detail below using silicon
carbide as a convenient and commercially important example. However, it
should be recognized, as it has been by the inventors, that articles
according to the invention may be made by practicing the process of the
invention using any material that can be deposited by CVD. Illustrative,
non-limiting examples include oxide, nitride and carbide ceramic
materials including but not limited to aluminum nitride, aluminum oxide,
aluminum oxy-nitride, silicon oxide, silicon nitride, silicon
oxy-nitride, boron nitride, boron carbide, and other materials such as
zinc sulfide, zinc selenide, silicon, diamond, diamond-like carbon, and
any other material that can be prepared using CVD techniques.

[0034]In addition to allowing production of flat, dimensionally stable
rings, the present invention allows for the production of doped SiC rings
that have improved opacity, resulting in reduced or minimal light
leakage. These rings are particularly suitable for use as support edge
rings in semiconductor apparatus that subject Si wafers (and the
supporting rings) to high temperature processing such as rapid thermal
processing, and for use in other apparatus that are subject to high
temperatures.

[0035]In this respect, the invention relates to a silicon carbide
material, comprising

[0036](a) CVD-deposited silicon carbide; and

[0037](b) about 100 ppm to about 5000 ppm nitrogen dispersed in the
CVD-deposited silicon carbide,

wherein the silicon carbide material has an opacity greater than that of
CVD-deposited silicon carbide.

[0038]The methods described above produce silicon carbide materials
containing grains having their growth direction oriented in the plane of
the material, and substantially parallel to each other. In addition, the
grains preferably exhibit rotational orientation that is substantially
random with respect to the axis of grain growth. For materials with cubic
crystal structure such as SiC, this results in a random x-ray diffraction
pattern when the plane of the part is measured. For non-cubic crystal
structure materials deposited by the present invention, it is believed
that at least some of the grains are also randomly oriented with respect
to the axis of grain growth. The result is a material that is flat,
dimensionally stable, uniform in surface appearance, and suitable for
coating with silicon layers, if necessary.

[0039]In this regard, the invention can be thought of as relating to a
planar structure comprising of a CVD deposited silicon carbide material
further comprising SiC grains having their axes of growth substantially
parallel to each other, and having rotational orientation that is
substantially random with respect to the axis of grain growth.
Alternatively, the invention relates to a CVD deposited silicon carbide
material comprising FCC Moissanite-3C silicon carbide having a 220 to 111
peak ratio between about 0.30 and about 1.25, most preferably between
about 0.33 and about 0.60, as measured by x-ray diffraction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a schematic of a CVD apparatus suitable for use in
practicing the invention.

[0041]FIG. 2 is a perspective view of a round cross-sectional tube for use
with the CVD apparatus of FIG. 1.

[0042]FIG. 3 is a side perspective schematic view of a segmented graphite
tube showing the gas injection flow through the tube.

[0043]FIG. 4 is a schematic view of the material formed by deposition on
the inside of the tube of FIGS. 2 and 3 as it is being sliced.

[0044]FIG. 5 is a schematic view showing the crystal growth of a ring
formed by the methods of the prior art.

[0045]FIG. 6 shows the crystal growth of a ring formed by the methods of
present invention.

[0046]FIG. 7 is a top view schematic showing the crystal growth of a flat
ring formed by the methods of present invention.

[0047]FIG. 8 is a schematic showing an alternate CVD apparatus suitable
for use in practicing the invention.

[0048]FIG. 9 is a schematic showing SiC ring blanks used to prepare bars
for MOR testing.

[0049]FIGS. 10a and 10b are optical bright field micrographs of etched
samples of material made according to one embodiment of the invention.

[0050]FIG. 11 is a schematic diagram of a rapid thermal processing
apparatus for semiconductor wafers using rings prepared according to one
embodiment of this invention.

[0051]FIG. 12 is a cross-sectional diagram illustrating an edge ring of
the rapid thermal processing apparatus of FIG. 11.

[0052]FIG. 13 is a micrograph of the microstructure of a prior art CVD SiC
ring, with observation perpendicular to the plane of the ring.

[0056]The largest market for SiC sheet material is the semiconductor
market, which uses the high stiffness to weight ratio, chemical and
physical compatibility and purity of CVD SiC to make fixtures to support
silicon and other wafers for processing. Many of these fixtures are
shaped as rings, typically less than 0.2'' in thickness and up to about
14'' in diameter. The inner diameter is adapted to hold semiconductor
wafers.

[0057]Although articles made according to the techniques described below
are primarily ring-shaped, it is understood that they may have any
desired shape, depending upon the size and the shape of the substrate
used to form the article. For ease of discussion, however, the below
description refers to flat ring-shaped articles and methods of forming
the ring-shaped articles using a cylindrical tube as the substrate.

[0058]The chemical vapor deposition (CVD) process for silicon carbide
(SiC) is generally based upon a thermally induced reaction within a
reduced pressure, resistance heated graphite furnace. Certain embodiments
of the invention use this equation and these basic parameters:

Si(CH3)Cl3(g)→SiC(s)+3HCl(g)

[0059]Deposition temperature: 1300-1400° C.

[0060]Deposition pressure: ˜200 torr

[0061]Catalyst: H2

[0062]Instead of using flat sided or four sided boxes for deposited
material to faun a sheet conforming to the dimensions of the sides of the
boxes, the invention provides at least one round cross-sectional tube,
the internal dimension of which accommodates the outer diameter of the
desired ring size. (If a triangular, oblong, oval, octagonal, or any
other shaped article is desired, it is also possible to use tubes having
the corresponding cross-section, although it is preferred that instead of
sharp corners, the substrates have rounded corners and edges to provide a
more uniform distribution and to reduce internal stresses.)

[0063]The apparatus used in the process of the present invention is a
chemical vapor deposition (CVD) chamber designed to deposit ceramic
material on the inside of a cylindrical or tubular mandrel. FIG. 1 is a
schematic diagram that shows a CVD system that includes a furnace chamber
10, a gas distribution system 12 in fluid communication with the furnace
chamber 10, a pumping system 14 in fluid communication with the gas
distribution system 12, a power supply 16, and an effluent treatment
system 18. The gas is pumped into and out of one or more box substrates
19.

[0064]In this embodiment, the invention uses as a substrate a round or
circular cross section cylindrical tube 20, as shown in FIGS. 2 and 3.
The cylindrical tube 20 is typically mounted in a vertical direction in
the chamber 10, although it could be mounted at any other angle.

[0065]In use, reactant gas is admitted at an injector (shown schematically
at 30 in FIG. 1) at the top of chamber 10 (although it could
alternatively or additionally come in from the bottom) and into
cylindrical tube 20. The gas is removed by an exhaust port (shown
schematically at 32) at the bottom of chamber 10 (although it could be at
the top) which connects to the pumping system 14 and effluent treatment
system 18.

[0066]In this embodiment, the cylindrical tubes 20 in FIG. 2 are
preferably graphite tubes that are attached in a non-rotating manner in
chamber 10. Cylindrical tubes 20 may be attached such that they are
allowed or caused to rotate, but rotation is not required and is
typically not employed.

[0067]The graphite cylindrical tubes 20 have a substantially round
cross-section, such that the material deposited on the inside of the
tubes is formed in a corresponding round cross-sectional shape. The
circular radius of cylindrical tubes 20 allows uniform distribution and
flow of gases. A release coating or other substance may be applied to the
inner surface of the graphite tube to facilitate removal of the CVD
deposited SiC material.

[0068]A cylindrical tube 20 that forms the substrate for the deposition
process is shown in FIGS. 2 and 3. Tube 20 can be a single, full length
tube, or it may be a series of tube sections 24 as shown in FIG. 3. If
the tube is graphite, which is typically used for the SiC CVD process, a
full length tube will be quite heavy and difficult to work with. Thus,
one option is to provide a tube that is actually a series of tubes, for
instance, four or five tubular sections, that can be attached with a
joining fixture. Because the resulting deposit that is formed will be
sliced or otherwise sectioned, the joints between the tube sections need
not be seamless. In fact, the joining fixture may be a device that
provides an indentation between the tube segments that acts as a
deposition inhibitor so that the tube segments can be readily separated
from one another. While any length of tube or the segments can be used,
in certain embodiments, the tube segments 24 are about twelve inches high
and about three to six segments are attached to one another, yielding a
cylindrical silicon carbide deposit that is about 40 to about 60 inches
high.

[0069]The tube 20 can have the same cross-sectional dimensions throughout,
or the cross-section can be varied. For example, if the deposition tends
to be greater at certain areas of the cylinder than at others, the
geometry of the cross-section can be varied to accommodate those
deposition variations. The cylinder can also be tapered if
differently-shaped disks are desired. Alternatively, the cylinder can be
terraced or stepped to provide clear divisions between the sections.

[0070]During processing, precursor gas is fed into the internal diameter
of the cylindrical tube 20 in order to make a tubular monolith of CVD
deposited material, such as a ceramic material. In the illustrated
embodiment, gases are fed at the top of the chamber through injector 30
and exhausted at a manifold or exhaust port 32 at the bottom of the
chamber. Each of the cylindrical tubes 20 has a dedicated, independent
injector or precursor gas feed positioned so that the gas enters at one
end 26 of cylinder and exits through the other end 28. This allows
deposition primarily on the surfaces of interest only, the internal
diameter 22 of the tube 20, and not on other surfaces, such as the
outside of the cylindrical tube 20. The gas travels in a path along the
axis of the tube. This provides a more efficient deposition process by
allowing better control of gas conditions. The only asymmetry in
thickness that may arise is a thickness gradient from the top to the
bottom of the tube, although the thicker areas can be machined to the
proper thicknesses.

[0071]In certain specific embodiments, the gas feed is a linear,
symmetrical chemical vapor deposition system with reaction gas entering
the chamber approximately at the center of the diameter such that the gas
is deposited substantially evenly along the inside surface 22 of the tube
20. The reaction product is deposited to a set thickness on the internal
surface. The SiC builds up on the inner surface of the tube in a
substantially radial direction, as shown by FIGS. 6 and 7.

[0072]This process is more efficient than the prior art process in which
reaction products are deposited throughout the chamber and not at
specific locations. The cylinder also does not need to be rotated in
order to obtain the desired uniform deposition in a consistent manner, in
contrast to many methods of the prior art, although it could be rotated
if desired.

[0073]Typically, the deposition thickness is approximately 1 inch,
although a wide range of material thickness from approximately 0.1 inch
to over 2 inches may be obtained by varying gas flow rates and deposition
times. Gases from multiple tubes may be evacuated commonly from the
chamber though exhaust port 32.

[0074]Once the silicon carbide has been deposited on the inside of the
tubular mandrel, the deposit is removed from the mandrel by any
appropriate technique, including, but not limited to grinding or
electrical discharge machining.

[0075]The resulting SiC tube 202 can be cut through the circular cross
section to yield a ring or a hoop 204, as shown in FIG. 4. Cutting can be
accomplished by using ceramic machining techniques including, but not
limited to, diamond slicing, ultrasonic cutting or laser cutting. In the
case of using an electrically-conductive deposited material, electrical
discharge machining can be used to slice rings from the tubular section.

[0076]Rings of varying diameter can be produced by varying the
cross-sectional width and thickness of deposition to give the desired
internal diameter. Overall height of the ring can be adjusted by varying
the height of cutting.

[0077]The outer diameter of the ring or hoop 202 will correspond to the
inner diameter of the cylindrical tube 20 and the inner diameter of the
ring or hoop will depend upon the amount of gases that are fed through
the tube and allowed to deposit. The thicker the deposit, the smaller the
inner diameter of the ring that can be produced. The diameters of the
ring may then be machined, smoothed or otherwise shaped to provide the
desired dimensions.

[0078]In an alternative embodiment, a SiC tube can be formed by depositing
the material on the outside of a cylindrical mandrel, again by CVD. In
this case, a release coating or other substance may be applied to the
outer surface of a tubular graphite mandrel to facilitate removal of the
CVD deposited SiC material. When the deposit reaches the desired
thickness, the mandrel can be removed and the resulting annular SiC
cylinder can be sectioned as described herein. It is believed that the
same or similar stress distribution properties as are obtained for
deposition on the inside of the tube will be obtained using this
alternative method, namely, that the stress distribution pattern will be
radially symmetric, leading to enhanced dimensional stability in rings
cut from this material.

[0079]An alternate embodiment that may be used to form rings is shown in
FIG. 8. In this embodiment, a rod 802 is disposed within the cylindrical
tube 803, and material, represented by arrows 804, is caused to deposit
both on the inner surface of the tube as well as on the outer surface of
the rod. This configuration may be used to produce differently sized
tubular CVD material deposits, and subsequently produce two
differently-sized rings; one ring that corresponds to the inner diameter
of the cylinder and another ring that corresponds to the outer diameter
of the rod.

[0080]Another alternative embodiment relates to the formation of
differently sized rings by depositing CVD SiC onto the inner and outer
surfaces of a tube disposed within the reactor, again giving two
differently sized tubes, which can then be cut to different diameter
annular rings. These embodiments are particularly suited to prepare rings
suitable as wafer supports in semiconductor furnaces, and in other
applications where different sized rings are desirable and/or necessary.
Another benefit to these techniques for producing rings is that they
result in matched sets of rings from the same deposition process, and
containing the same material. This is desirable when qualifying new
materials for use in expensive semiconductor processing apparatus, since
only one material produced by a single process is being qualified.

[0081]Rings sliced from cylindrical deposits of this invention are easier
to machine than rings made using the methods of the prior art due to the
lower internal or out-of-plane stresses around the ring. Generally, as
material is deposited via CVD, there are subtle but unintentional
variations in the process parameters that can affect the nature of the
material and the crystal growth structure can vary, creating inherent
stresses in the material. These stresses can cause bowing or bending in
rings that are cut out of a sheet of material with the predominant
direction of crystal growth in the direction normal to the plane of the
sheet. By contrast, the symmetry provided by using a cylindrical tube
mandrel or support according to the invention affords a substantially
symmetrical stress gradient around the ring, so that any inherent stress
exhibited on one side of the ring is balanced by a similar inherent
stress on the opposite side of the ring (i.e., there is no net stress in
the plane of the ring). In other words, the inherent stress is directed
inwards or outwards in a similar manner, affecting the ring or disk in a
substantially symmetrical way.

[0082]For SiC that is deposited upon a flat surface, the stress gradient
on the lower surface 44 will be different from the stress gradient on the
upper surface 46 due to the way the ring is manufactured. This difference
in stress gradient can cause the material to bend or bow, preventing the
manufacture of a flat ring.

[0083]By contrast, for SiC that is deposited within a cylindrical tube,
the stress gradient difference is exhibited between the outer surface 48
and in the inner surface 50, as shown in FIGS. 6 and 7, such that any
stress gradient "squeezes" the ring symmetrically. Accordingly, rings
that are cut from the resulting cylindrical deposition have substantially
uniform properties around them due to the uniformity of the deposition
parameters across that particular slice. (However, it is possible that
properties of slices cut from different positions along the cylinder will
be slightly different due to the different deposition parameters along
the particular length of the cylinder.)

[0084]If the cylindrical tube 20 is provided as a series of sections, the
deposit can be removed by disassembling the sections. Even if the
resulting deposit fractures or cracks, the cracked areas do not cause a
great deal of wasted material because they will typically be across the
diameter of the material due to the microcrystalline structure of the
material. This is the portion of the ring that will be machined during
cutting or slicing.

[0085]During the prior art deposition process (whether the ring is formed
from a flat sheet or on a ring shaped mandrel), the growth direction of
the crystal structure is in the direction of the height 40 of the ring,
as shown in FIG. 5. This is the case whether the deposit is formed on a
flat graphite sheet, on a series circular mandrels mounted on a shaft, or
on graphite rings suspended in the chamber.

[0086]However, rings according to the present invention exhibit the growth
direction of the crystal structure in the direction of the length 42 or
diameter of the ring, as shown in FIGS. 6 and 7. In other words, the
crystal growth forms on the inside of the cylindrical tube 20 and extends
inward in a substantially radial manner. When the resulting cylindrical
deposit that is formed is removed from the tube, it will have a long axis
and a circumference. The growth of the crystal grain is around the radial
circumference and perpendicular to the long axis. Once the cylindrical
deposit is sliced or cut into individual rings, the grains are oriented
in the plane of the finished article. This is directly contrary to the
crystal growth seen in rings that are manufactured using traditional
methods.

[0087]This can be seen more clearly by reference to FIGS. 12, 13, 14, 15
and 16, which illustrate the microstructural differences observed in edge
rings machined from CVD SiC prepared by the process of the invention and
by the conventional prior art processes. FIG. 12 provides context by
showing what an edge ring is, and how it is used. FIG. 12 is a
cross-sectional diagram of several elements of the rapid thermal
processing system shown in FIG. 11, in which silicon wafer 52 is held at
the outer diameter by edge ring 54 mounted horizontally on top of one end
of vertical support cylinder 56. Edge ring 54 may comprise a CVD SiC ring
which may optionally be coated with silicon layer 58. Edge ring 54 may be
machined with a recess to hold the wafer as shown in FIG. 12. Ring 54 may
be incorporate other geometrical features (not shown) depending on the
specifics of the design criteria.

[0088]FIG. 13 is a top-down view optical micrograph of the microstructure
of a prior art CVD SiC ring, with observation perpendicular to the plane
of the ring. This ring was prepared from a sheet of CVD SiC manufactured
by the prior art process of Example 1. The specimen was etched to reveal
the grain structure within the SiC material. The orientation of the SiC
grains is random within the plane of the ring.

[0089]FIGS. 14a and 14b are 50× magnification optical micrographs
obtained from metallographic cross-sections of a prior art CVD SiC edge
ring that was coated on both sides with CVD Silicon, corresponding to the
horizontal flat sections of FIG. 12. The specimen was not etched, but the
image was taken with polarized light to accentuate the grain structure
within the CVD SiC material. The CVD SiC material was produced by the
process of Example 1. The arrows indicate the direction parallel to the
plane of the ring, pointing from the outer diameter toward the inner
diameter of the ring. The vertical orientation of the long axis of the
SiC grains is clearly visible, which clearly shows that the direction of
growth of the SiC grains is perpendicular to the plane of the ring.

[0090]FIG. 15 is a top-down view optical micrograph of the microstructure
of a CVD SiC ring of the present invention, with observation
perpendicular to the plane of the ring. The ring was sliced from a CVD
SiC tube prepared by the process of Example 2 of the present invention.
The specimen was etched to reveal the grain structure within the SiC
material. The arrow shows the direction parallel to the plane of the
ring, pointing from the outer diameter toward the inner diameter of the
ring. This micrograph clearly shows that the SiC grains are oriented
parallel to the plane of the ring. It is also observed that the grains
are oriented along the radius of the ring. This grain structure is in
sharp contrast with the microstructure of prior art CVD SiC rings shown
in FIG. 13.

[0091]FIGS. 16a, 16b, 16c and 16d are 50× magnification optical
micrographs obtained from metallographic cross-sections of a CVD SiC edge
rings of the present invention that were coated on both sides with CVD
Silicon, corresponding to the horizontal flat sections of FIG. 12. The
specimens in FIGS. 16a, 16b and 16c were not etched, but the images were
taken with polarized light to accentuate the grain structure within the
CVD SiC material. The specimen in FIG. 16d was etched in a dilute mixture
of hydrofluoric acid and nitric acid to reveal the microstructure of the
CVD Si coating. The CVD SiC material was produced by the process of
Example 2. The arrows indicate the direction parallel to the plane of the
ring, pointing from the outer diameter toward the inner diameter of the
ring. Note the horizontal orientation of the long axis of the SiC grains,
which clearly shows that the direction of growth of the SiC grains is
parallel to the plane of the ring. This grain structure is in sharp
contrast to the microstructure of the prior art SiC ring shown in FIG.
14. FIG. 16d clearly shows that the direction of growth of grains within
the CVD Silicon coating is perpendicular to the plane of the ring, and
perpendicular to the direction of the SiC grains as well.

[0092]Measurements of the grain size and grain aspect ratio were obtained
for various samples of rings obtained from Example 2. Only grains having
a dimension of greater than 10 μm were measured. These grains
exhibited aspect ratios ranging from about 2 to about 10, more typically
about 3 to about 6. The photomicrograph in FIG. 15 shows that the grains
are highly oriented in the plane of the ring, but are otherwise randomly
oriented. In other words, the grains line up so that their long axes are
substantially parallel to the ring plane. However, the grains themselves
exhibit rotational orientation that is substantially random with respect
to the direction of grain growth. Without wishing to be bound by any
theory, it is believed that this combination of radial, planar
orientation of grain growth axis and substantially random orientation of
the grains with respect to the grain growth axis is what provides the
benefits of symmetrical stress distribution described above, and also
avoids fluctuations in the surface profile of the rings.

[0093]In one embodiment of the method of the present invention,
free-standing silicon carbide tubes from which the rings are sliced are
deposited from a mixture of silicon carbide precursor gases, such as a
mixture of methyltrichlorosilane (MTS) and hydrogen, with an optional
inert gas, such as argon or helium and optional dopant gas into a
deposition chamber heated to a temperature typically between 1300°
C. and 1400° C. Deposition pressure is maintained at approximately
100 ton to 300 torr. The relative partial pressure flow ratio of hydrogen
to MTS is maintained in the range of approximately 5 to 10.

[0094]Reaction gases are delivered into a tubular mandrel through a single
injector or an array of injectors positioned symmetrically at either the
top or bottom. Under these conditions, silicon carbide is deposited on
the inside of the tubular mandrel(s) at a deposition rate of
approximately 1 to 3 μm/minute and the deposition process is continued
until the desired thickness of SiC material is achieved. Typical
deposition periods vary between 50 and 300 hours.

[0095]Upon completion of the deposition, the mandrel and deposit are
removed from the reaction chamber and with or without first separating
the mandrel from the deposited material, the silicon carbide rings are
sliced from the tube. The outer and inner dimensions and the thickness of
the ring are then machined to specification using diamond grinding
methods.

[0096]The unique characteristics of the products of the present invention,
i.e. low in-plane residual stress, extreme flatness, high control of
machinability, allow for the manufacture of very thin (e.g. less than 0.5
inch) hoop-shaped rings, and more complicated rings with or without bulk
"drop-offs."

[0097]The process according to the invention can also be adapted to
provide extremely opaque wafer support rings. As shown in FIG. 11, wafer
support rings support a Si wafer in a rapid thermal processing reactor
where the wafer is heated from above to relative high, but highly
controlled, temperatures. In order to control temperature effectively,
accurate pyrometery measurements are very important. The Si wafer is
illuminated from above by tungsten halogen lamps 60, disposed in light
pipes in a water cooled housing 62. Light passes through quartz window 64
and onto the Si wafer. Because standard CVD SiC rings have different
optical characteristics from those of the Si wafer, pyrometery
observations, which are taken from below the wafer by fiber optic probes
66 and reflector plate 68, can be corrupted by light leakage through the
edge rings 54, which are supported by cylindrical support 56.
Accordingly, there is a desire to produce support rings with an opacity
similar to that of the Si wafer. This can be done by coating the SiC
rings with a relatively thick (75-100 micron) layer of Si.

[0098]However, another solution to increasing opacity of the rings is to
dope the CVD SiC with a suitable dopant. One such dopant is nitrogen in
amounts ranging from about 100 to about 5000 ppm. The nitrogen gas is
simply added to the gas mixture during deposition of the rings, as
described above. This process increases opacity of the rings by a factor
of approximately 10,000 to 10,000,000 times, producing a SiC material
that is considerably closer to Si in optical transmission properties, and
reducing light leakage, thereby reducing the need for coating with a Si
layer. Another suitable dopant is boron.

EXAMPLES

[0099]Using the process of the present invention, 200 mm and 300 mm
diameter SiC edge rings for Si wafer processing have been produced and
characterized. Rings were made from both electrically non-conducting
CVD-deposited SiC material exhibiting a typical electrical resistivity of
about 100 to 10,000 ohm-cm, and electrically conductive SiC with an
electrical resistivity of less than 0.01 ohm-cm.

[0100]In the following Examples, flexural strength measurements, also
known as modulus of rupture (MOR) tests, on the deposited SiC materials
were obtained using the 4-point bend test according to ASTM C-1161. The
MOR test samples (bars) were cut from rings sliced from the deposited
cylinders. The MOR bar samples had the following dimensions: length of 45
mm, width of 4±0.13 mm, thickness of 3±0.13 mm, edge chamfer of
0.12±0.03 mm at an angle of 45°±5°. Chamfers were
fabricated by grinding with a 600-grit grinding wheel, while the large
planar sides of the MOR bars were finished with a 320-grit wheel. The
approximate layout of the MOR bars 9a cut from the SiC ring blank 9b is
shown in FIG. 9. The 44 mm×4 mm side of the MOR bar was obtained
from the plane of the ring.

[0101]Electrical resistivity measurements were made with a Jandel Company
hand held 4-point probe, consisting of a linear array of 4 points spaced
1 mm apart. The resistivity was calculated assuming an infinite material
thickness. Three data sets were obtained at each of four measured
positions on each sample ring.

Example 1

Comparative

[0102]SiC rings were fabricated as follows. A graphite box of size
approximately 15 inches×15 inches×50 inches was formed from
four isostatically pressed, fine grained graphite plates having a thermal
expansion similar to that of dense CVD SiC was assembled and installed in
a CVD reactor.

[0103]Deposition gases hydrogen and MTS were introduced through an
injector positioned at the top of the box assembly at a ratio of
hydrogen-to-MTS approximately 8, and a pressure of 200 torr. Chemical
vapor deposition of SiC was carried out at approximately 1350° C.
The total deposition time was approximately 200 hours, with a typical
deposition rate of the SiC material of 0.002 inch per hour.

[0104]After deposition, the reaction chamber was opened and the sheet SiC
deposits were removed. Each of the sheet SiC deposits was cut into ring
blanks having a thickness of approximately 0.2 inch. These slices were
then ground to the final ring specification.

[0105]The SiC material deposited by this technique had the following
typical values: a density of 3.21 g cm-3, Vickers hardness >2700
kg mm-2, thermal conductivity of 250 Wm-1K-1, electrical
resistivity between 100-10,000 ohm-cm, and flexural strength in the range
of 45 to 70 ksi (310 to 480 MPa).

Example 2

[0106]SiC edge rings for handling 300 mm Si wafers for semiconductor
processing were fabricated as follows. A graphite tube with nominal inner
diameter of 340 mm was manufactured from an isostatically pressed, fine
grained grade of graphite with a thermal expansion similar to that of
dense CVD SiC. Four tubular sections, each with a length of approximately
13 inches were assembled in a CVD reactor. Optional
deposition-prohibiting spacer devices were positioned in between each
section to facilitate removal of each tubular section after deposition.

[0107]The precursor gas MTS was introduced through a single injector
positioned symmetrically at the top of the tubular mandrel assembly at a
flow rate of 9.1 liters per minute. Hydrogen was delivered through the
same injector at a flow rate of 76 liters per minute. The ratio of
hydrogen-to-MTS was 8.4. The CVD reactor pressure was 200 torr and
deposition temperature was 1350° C. The total deposition time was
174 hours, and the average deposition rate of the SiC material was 0.005
inch per hour.

[0108]After deposition, the reaction chamber was opened and the tubular
mandrels, spacer devices and tubular SiC deposits were removed. Each of
the tubular SiC deposits was then sliced using a diamond abrasive
slitting wheel into ring slices having a thickness of approximately 0.2
inch. These slices were then ground to the final edge ring specification.

[0109]The SiC material deposited by this technique had a density of 3.21 g
cm-3, Vickers hardness >2700 kg mm-2 and thermal
conductivity between 240 and 270 Wm-1K-1, and an electrical
resistivity between 390 and 450 ohm-cm. These measurements are in the
same range as Jandel probe resistivity measurements obtained from
plate-form SiC material that has been deposited under similar conditions,
but on flat graphite sheets.

[0110]The flexural strength of rings obtained by this process varied
between 40 and 65 ksi (275 to 450 MPa), depending upon where the ring was
sliced from the tubular deposit. FIG. 10 presents bright field optical
micrographs of an etched sample obtained from a ring which exhibited a
flexural strength of 62 ksi. The lower portion of FIG. 10(a) is the outer
diameter of the ring slice, which is part of the deposit that was next to
the graphite mandrel substrate. The upper portion of FIG. 10(b) shows a
portion of the deposited material taken from a point 13.2 mm from the
substrate, i.e. the SiC deposit was 13.2 mm thick. The increase in grain
size within the SiC deposit as the distance from the substrate is
increased is clearly noticeable, as is the orientation of the grains as
illustrated in FIG. 6.

[0111]That the rings of the present invention with a SiC grain growth
orientation parallel to the plane of the ring had a flexural strength
equal to that of rings made from SiC material with grain growth
orientation perpendicular to the plane of the ring was surprising. This
excellent result was unexpected since in the flexural strength test of
the material of the present invention, the SiC material is bent in a
direction parallel to the direction of grain growth, i.e. using an
analogy to wood, the material is bent "along the direction of the grain."
Therefore, the present invention overcomes prior art deficiencies
associated with fabrication of ceramic rings, while maintaining
outstanding material strength.

Example 3

[0112]SiC edge rings for handling 200 mm Si wafers for semiconductor
processing were fabricated according to the process of Example 1, with
the following exceptions: the graphite tube had a nominal inner diameter
of 240 mm, each tubular graphite section had a length of approximately 9
inches, and the hydrogen-to-MTS flow ratio was 7.4. The average
deposition rate was 0.0051 inch per hour. The flexural strength of rings
obtained varied between 57 and 68 ksi (390 to 465 MPa), again indicating
outstanding material strength.

Example 4

[0113]SiC edge rings for handling 200 mm Si wafers for semiconductor
processing were fabricated according to the process of Example 2, with
the following exceptions: each tubular graphite section had a length of
approximately 8.5 inches, and the hydrogen-to-MTS flow ratio was 8.4 and
a nitrogen doping gas was added to produce an electrically conducting SiC
deposit. The average deposition rate was 0.0065 inch per hour. Rings
obtained varied in electrical resistivity from 0.004 ohm-cm to 0.007
ohm-cm. For comparison, SiC material deposited in plate form that has
been deposited under similar conditions onto flat graphite sheet
typically exhibits a resistivity between 0.002 ohm-cm and 0.008 ohm-cm.
Therefore, the electrical resistivity measurements obtained from the
deposit in Example 4 are similar to measurements obtained from plate-form
SiC material deposited under similar conditions.

[0114]Representative rings from the prior examples have been coated with
CVD silicon coatings to reduce opacity to light and to provide a high
purity Si surface for contact to Si wafers during wafer processing.
Typically, this deposition is conducted at or near atmospheric pressure
using trichlorosilane as a source of silicon plus hydrogen as a diluent
and reaction catalyst gas and a CVD temperature of between about
1000° C. and 1200° C. Other vacuum deposition and vapor
deposition methods including but not limited to evaporation, sputtering,
ion plating, ion beam deposition and plasma deposition can be used to
deposit coatings over the products of the present invention. Because of
the orientation and uniform nature of the microstructure of the material
of the present invention, coatings applied to the products of the present
invention have improved cosmetic appearance compared with coatings
applied to prior art CVD products, e.g. the CVD Si-coated SiC rings of
the present invention do not display variation in cosmetic appearance, as
further explained below.

[0115]The microstructure of SiC rings made from the standard "sheet" SiC
material (using conditions similar to Example 1) and the process of the
present invention (Example 2), and the effect of the microstructure of
these materials on the structure and appearance of deposited Si coatings
were evaluated. The materials were characterized by optical microscopy
and X-ray diffraction (XRD). X-ray diffraction measurements were
performed with the x-ray beam incident on the large flat planar surface
of the ring samples.

[0116]Measurement of Si coating structure was performed by plane-view
optical microscopy examination at 100×-200× magnification.
XRD texture analysis was performed on a Philips X'Pert MPD diffractometer
that was set up with parallel beam optics using Cu radiation operating at
45 kV and 40 mA. Tests were run over the angular range of 25 degrees to
90 degrees to capture all of the principal diffraction peaks from SiC and
Si.

[0117]For the XRD analysis of each sample, the Si top layer was scanned
first to obtain the texture of the Si layer. Then, the Si coating was
completely removed by immersing the sample in a 5% solution of
hydrofluoric acid in nitric acid for 1.5 hours. Then, a second scan of
the underlying SiC ring substrate was obtained. In this way, XRD data was
obtained on the Si coating and the SiC substrate from the same location
on the part.

[0118]Several Si-coated CVD SiC rings were chosen for analysis based on
the method of manufacturing and the cosmetic appearance of the Si
coating. The samples and analysis results are presented in Table 1,
wherein Sample ring 80C2-01 was obtained from a ring made by the process
of Example 2 of the present invention, and Samples 45CN113 and 67CN463
were made from sheet form CVD SiC by processes similar to Example 1.
There was no variation in cosmetic appearance of any areas of Sample
80C2-01, therefore Sample 80C2-01-4 is representative of the entire ring.
However, samples 45CN113-1 and 45CN113-2 were two samples cut from
different areas of the same ring with different cosmetic appearance.
Sample 45CN113-2 was taken from an area of the ring that exhibited a
uniform appearance, and Sample 45CN113-1 was taken from an area of the
ring that exhibited a non-uniform appearance. Likewise, Samples 67CN463-4
and 67CN463-5 were two samples cut from areas of the same ring with
different cosmetic appearance. In this case, Sample 67CN463-4 appeared
slightly non-uniform, and Sample 67CN463-5 was very non-uniform. The
following discussion illustrates the relationship between the uniformity
of cosmetic appearance of the silicon coating and the 220/111 peak ratios
obtained from x-ray diffraction measurements of the underlying SiC
material for these samples.

The XRD texture was essentially the same for all of the Si coatings on the
samples in Table 1. By comparison with the powder diffraction pattern
database, all coatings were the cubic phase of silicon. Each spectrum
showed an extremely strong 220 preferred orientation, although some of
the other Si peaks were barely visible at levels less than 1%.

[0119]Table 1 also presents the XRD texture results from the same samples.
By comparison with the powder diffraction pattern database, the SiC
structure for all samples was the face-centered cubic phase Moissanite-3C
SiC structure. According to the powder diffraction pattern database, the
ratio of SiC 220 to SiC 111 of approximately 1:3 is indicative of a
randomly oriented SiC material of the Moissanite-3C structure. Sample
80C201-4 of the present invention is seen to have a completely random XRD
texture, virtually identical to the powder diffraction database pattern,
while the other samples displayed either a minor preferred or a
significant preferred 220 orientation. Sample 45CN-113-2 was almost
random with a slight 220 oriented component, while the texture in sample
45CN113-1 is strongly 220 oriented. Sample 67CN463-4 exhibited a
preferred 220 orientation with a significant random component, whereas
the texture of Sample 67CN463-5 was very strongly 220 oriented.

[0120]There is an apparent relationship between the 220/111 peak ratios of
the SiC materials obtained by x-ray diffraction measurements and the
cosmetic appearance of the Si coating. The low 220/111 ratio for Sample
80C2-01-4 is consistent across the ring, resulting in a highly uniform Si
coating appearance. By contrast, the higher and variable 220/111 peak
ratios obtained from the prior art samples of ring 45CN133 and 67CN463
results in a non-uniform appearance of the Si coating.

[0121]Observations of the silicon coated surfaces indicated that a
preferred 220 XRD texture in the SiC material leads to the growth of
larger grains in the CVD Si coating. The reason for this is not
understood, but may be due to the close lattice match between 220 Si and
220 SiC. Variation in the degree of 220 orientation on different
locations of the SiC ring leads to variation in the crystal size of the
deposited CVD Si coating, resulting in a non-uniform appearance in
reflected light. The SiC ring made by the process of the present
invention displays a randomly oriented texture which results in a CVD Si
coating of highly uniform appearance.

[0122]The inventors also believe that improved cosmetic appearance with
the SiC rings of the present invention may be due to the almost
perpendicular orientation of the grains in the Si coating compared to the
SiC grains in the ring material. This is in sharp contrast to prior art
processes in which the grain structure of the Si coating is oriented in
the same direction as the grain structure of the SiC ring material.

[0123]It has also been observed that the SiC rings of the present
invention have improved optical opacity compared to prior art CVD SiC
rings. It is believed this improved opacity may also be related to the
random orientation of the crystal structure in the plane of the ring
compared to the preferred 220 orientation of prior art CVD SiC ring
materials.

[0124]In addition to the excellent material properties summarized above,
additional unexpected benefits have been found with regard to processing
the materials of the present invention. In addition to providing
materials having better stress distributions and flatter profiles, the
method of the present invention also increases manufacturing capacity
significantly by allowing multiple cylindrical deposition chambers inside
one CVD furnace, replacing at least one of the conventional
squared-sectioned box sites or flat mandrels with at least one
cylindrical tube. In addition, other time and cost savings are realized
by use of the process of the invention.

[0125]First, when compared to fabrication of rings from plate-form CVD SiC
material, the fabrication time of product of the present invention is
significantly less. Typical fabrication, i.e. cutting or slicing the ring
from the deposit, then grinding to final dimensions, is reduced by
approximately 35%.

[0126]Also, because the rings are flat as sliced from the tubular deposit,
grinding to the specified ring flatness of 0.002 inch can be achieved on
the first attempt with the product of the present invention. In contrast,
plate-form deposited SiC material has a characteristic bow that must be
machined out to achieve a final flat part. Multiple processes of
alternate side grinding are required with plate-form SiC, and the first
grinds are outside of flatness specification be several thousandths of an
inch.

[0127]It has also been surprisingly found that the process of the present
invention produces a material that is very uniform and provides
consistent results during grinding, when compared to plate-form material.
The plate-form SiC material is often inconsistent in thickness and
uniformity across the dimension of a ring, and requires significant
machine operator judgment and adjustment to produce finished parts within
the desired product specifications. In addition, it is believed that the
non-uniform machining of the plate-form material can induce unwanted
stresses. Because of the uniform microstructure in the axial direction,
and because of the grain orientation of the present invention, the
material grinding to final configuration is much more consistent.

[0128]Although the specific examples presented above are for CVD silicon
carbide materials, similar results are anticipated with other materials
that can be deposited by CVD including, but not limited to other oxide,
nitride and carbide ceramic materials such as aluminum oxide, aluminum
nitride, aluminum oxy-nitride, silicon oxide, silicon nitride, silicon
oxy-nitride, boron nitride, boron carbide, and other materials such as
zinc sulfide, zinc selenide, silicon and diamond.